Many types of biometric features can be used as a form of identification and access control, while others may be used to monitoring physical status of a human body, such as heart bit, oxygen saturation, etc. Among methods of fetching biometric features, measuring impedance, including resistance and capacitance, of some part of human body is a convenient method in some instances. Some biometric module, such as a fingerprint sensing module, is designed to measure difference of capacitance or resistance over a sensing surface to fetch biometric data. An additional excitation signal source close to a sensing unit in the biometric module can be used to improve the performance of the biometric module. For example, in U.S. Pat. No. 8,736,001, a bezel structure coupled to circuitry to serve as a drive electrode for providing the excitation signals to a finger of a user is provided. In fact, like iPhone 6 marketed by Apple Inc, a metal ring is formed around the home key where a fingerprint sensor is embedded. The metal ring plays the same role as the bezel structure in '001. However, such technique may have two problems. First, the exposed bezel structure or the metal ring (called the signaling structure hereinafter) directly contacted with human body may cause malfunction of the fingerprint sensor. More specifically, size of the human body makes itself an antenna like device that can pick up radiation signals which may interfere with the fingerprint sensing function. Second, the signaling structure increases total height of a mobile device equipped with the biometric module and also causes a non-flat top surface of the device. It is not the compact and modern simplicity design that people are looking for.
Therefore, a trend is to absorb the signaling structure in a package level, i.e., Printed Circuit Board (PCB) level. Namely, it is to form the signaling structure as a step of a Printed Circuit Board Assembly (PCBA). Take a fingerprint sensing module for illustration, the PCBA shown in FIG. 1 will solve the problems mentioned above. However, since the fingerprint sensing module is made by different materials (the sensing unit may be an Integrated Circuit, the excitation signal source may be a specific metal and a protective layer would be a thermosetting resin), some unexpected trouble, such as structure deformation, would occur during the manufacturing processes.
Please see FIG. 1. FIG. 1 is a cross-sectional view of a fingerprint sensing module 1 formed by the above method. In order to have a clear view of the architecture of the fingerprint sensing module 1, scale in the vertical direction is larger than that in the horizontal direction. Namely, a real fingerprint sensing module is thinner than the fingerprint sensing module 1 in FIG. 1. The fingerprint sensing module 1 is basically composed of a PCB 2, a fingerprint sensing chip 3, two electrodes 4 and a protective layer 5. The PCB 2 functions as a substrate to carry all necessary electronic components of the fingerprint sensing module 1, including the fingerprint sensing chip 3 and the electrodes 4. The electrodes 4 are mounted on the PCB 2 and are very close to the fingerprint sensing chip 3. In fact, the electrodes 4 may be two separate metal bars, or in the form of a complete metal ring which is cut in two portions in the cross-section. The protective layer 5 may be made of a molding compound, spreading over the fingerprint sensing chip 3, the electrodes 4, other electronic components and partial surface of the PCB 2. The protective layer 5 is used to protect the elements underneath. The whole fingerprint sensing module 1 may be partially assembled in a secure device or a smart phone.
It is clear that the electrodes 4 and the protective layer 5 are made of different materials. The electrodes 4 may be made of a specific metal or alloy, e.g. aluminum. The protective layer 5 can mainly contain epoxy resin. When the materials of the protective layer 5 are applied and before the protective layer 5 is formed, a curing process must be done to heat up the fingerprint sensing module 1 to a temperature the epoxy resin can be fixed. Thermal expansion coefficient of the electrode 4 is much larger than that of the epoxy resin. When being heated, the electrode 4 expands to all directions as the solid arrows shown while the epoxy resin is losing liquidity and becomes fixed. Therefore, there are cracks in the protective layer 5 growing from the electrodes 4 while the fingerprint sensing module 1 is cooled back to room temperature. Besides, there may also be a reflow step to mount the fingerprint sensing module 1 to other electrical device. Those cracks may result in mechanical weakness and/or structure deformation of the module.
Therefore, a PCBA which forms an enhanced biometric module and a method for manufacturing the PCBA is desired to settle the problem mentioned above.